1. Field of the Invention
The present invention is related to pressure sensors, and in particular, to pressure sensors useable in robotic devices.
2. Description of the Related Art
Robotic systems often incorporate a robotic grasping mechanism. The robotic grasping mechanism is used to pick up and physically manipulate a work piece. In order to perform many desired manipulation operations, it is necessary to measure the pressure or force exerted by the grasping mechanism on the work piece, the contour of the pressure or force, as well as to measure any sliding motion of the work piece while in the grasping mechanism. Conventional pressure measuring solutions have been cumbersome and expensive, while often failing to provide sufficient pressure information.
For example, microswitches embedded on robotic fingers have been used to sense pressure. However, a microswitch, which is generally an on/off device, typically only indicates that pressure is being exerted on the microswitch, but does not provide information on the amount of pressure. In addition, hundreds or thousands of microswitches may be needed to provide sufficient pressure contour information. The associated wiring and expense makes the use of microswitches impractical in many applications.
Alternatively, hundreds or thousands of capacitive pressure sensors, each employing two metal plates separated by a layer of nonconductive foam and a parallel inductor, have been used to sense pressure. Unfortunately, a capacitive pressure sensor can give misleading readings when a good electrical conductor comes very close to the plates, changing the capacitance even if physical contact is not made. Thus, the capacitive pressure sensor may misinterpret proximity as pressure.
In an attempt to solve some of the problems associated with the capacitive pressure sensors described above, elastomer pressure sensors have been used in robotic applications. The elastomer is a foam pad with resistance that varies depending on how much it is compressed. An array of electrodes is connected to the top of the pad and an identical array is connected to the bottom of the pad to provide corresponding mates to the top electrodes. Each electrode in the top array receives a negative voltage, and its mate in the bottom array receives a positive voltage. When pressure appears at some point on the pad, the material compresses at and near that point, reducing the resistance between certain electrode pairs. This causes a current increase in a particular region in the pad. The location of the pressure can be determined according to which electrode pairs experience the increase in current. The extent of the pressure can be determined by how much the current increases. However, often hundreds or thousands of elastomer pressure sensors are needed to provide an adequate sensing surface for sensing pressure contours. In addition, the top and bottom arrays need to be precisely aligned for the sensor to operate properly. The associated expense and cumbersome interconnect wiring makes the use of such sensor impractical for many applications.
The present invention provides a novel optical pressure sensor useable in robotic applications, such as in conjunction with robotic grasping devices.
In one embodiment, an optical sensor is positioned to view the interior of a portion of a structure useable as part of a robotic grasping device, and to observe optically visible changes resulting from the exertion of pressure by or on the structure or on materials covering the structure. The images can then be used to determine the location and amount of pressure, as well as changes in the amount and location of pressure.
In one preferred embodiment, an optical sensor is positioned to view the interior of a substantially transparent structure forming at least a portion of a grasping device. The substantially transparent structure is at least partly covered by a base material having raised protrusions or projections thereon. The raised protrusions are positioned between the base material and a wall of the transparent structure. The raised protrusions visibly compress in response to pressure, such as the pressure resulting from the grasping device grasping an object. The optical sensor captures, through the transparent structure, images of the visible compression, and transmits the images to a processor for pressure determinations.
In one embodiment, the transparent structure is tubular in shape. In another embodiment, the transparent structure is spherical in shape. In yet another embodiment, the transparent structure is flat in shape. Optionally, the protrusions are formed from a foam material. The protrusions can be one of a pyramid shape, a spherical shape, a column shape, a conical shaped, a rod shape, and a complex shape. The optical sensor can include a lens to aid in viewing the protrusions.
In one embodiment, the processor executes a software module that utilizes the compression images from the optical sensor to determine the pressure exerted on at least one protrusion.
In yet another embodiment, a robotic grasping device includes a structure made of a resilient material with markings thereon. The structure deforms in response to pressure exerted on the structure. An image sensor views the markings, wherein the view changes as the structure is deformed. The image sensor captures images of the deformation, and transmits the images to a processor for pressure determinations.
In still another embodiment, a material that changes its optical characteristics in response to pressure or strain is used to sense pressure. For example, a coating, film or membrane of cholesteric liquid crystals is applied to, or overlays the sensor structure. For example, a coating, film or membrane of cholesteric liquid crystals is applied to, or overlays the sensor structure. When the structure, and hence the film, is subject to pressure, the patterns of internally reflected light off the film, as viewed by the image sensor, will change. The greater the pressure or strain, the greater the change in the film""s optical properties. Based on the amount and location of change in reflected light patterns, the pressure determination software module determines where pressure is being applied and how much pressure is being applied.
In one embodiment, polarized light is used to sense pressure. The light may be polarized by a transparent plastic sensor structure, via a polarizing filter placed over a light source, or the light can be emitted from an intrinsically polarized light source. The polarized light is emitted into the sensor structure. The structure is provided with an outer mirrored or reflective coating. A polarizing filter overlies the image sensor. The pressure and strain patterns within the sensor structure are rendered visible under optical polarization, and so can be imaged by the image sensor.